This non-provisional application claims priority under 35 U.S.C. xc2xa7119(a) on patent application No. 2002-095987 filed in Japan on Mar. 29, 2002, which is herein incorporated by reference.
1. Field of the Invention
The present invention relates to a NOx cleaning apparatus and a NOx cleaning method for an internal combustion engine provided with a NOx catalyst having NOx cleaning operation of exhaust gas under an oxygen rich atmosphere.
2. Description of Related Art
There is known a NOx cleaning apparatus using ammonia generated by hydrolysis by adding urea water into exhaust gas or adding ammonia directly to exhaust gas as a reducing agent of a selective reduction catalyst. When such a NOx cleaning apparatus is mounted to a vehicle having an internal combustion engine, the apparatus is constituted such that a selective reduction catalyst, known as an urea SCR catalyst (hereinafter, referred to as xe2x80x98NOx catalystxe2x80x99) capable of cleaning NOx of exhaust gas under an oxygen rich atmosphere, is arranged at an exhaust system of the internal combustion engine and urea water or ammonia is added to exhaust gas from an upstream side of the catalyst and supplied to the NOx catalyst as a reducing agent.
Generally, a NOx cleaning rate in a NOx cleaning apparatus is much dependent on the performance of the NOx catalyst, and the above-described NOx catalyst constituting a reducing agent by ammonia is provided with a property of processing to reduce NOx at a high efficiency in a region of the temperature exceeding 350xc2x0 C.
When urea water is added to exhaust gas, ammonia (NH3) is generated by being subjected to hydrolysis and thermolysis as shown by Equation (1).
(NH2)2CO+H2Oxe2x86x922NH3+CO2xe2x80x83xe2x80x83(1)
According to a denitrification between ammonia (NH3) and a nitrogen oxide (NOx) on the NOx catalyst, the reaction is carried out respectively in accordance with high or low of catalyst temperature, that is, mainly by Equation (2) at high temperature and mainly by Equation (3) at low temperature.
4NH3+4NO+O2xe2x86x924N2+6H2Oxe2x80x83xe2x80x83(2)
2NH3+NO+NO2xe2x86x922N2+3H2Oxe2x80x83xe2x80x83(3)
According to such a NOx catalyst constituting a reducing agent by ammonia, as shown by a bold line in FIG. 8, the adsorption amount of ammonia to the NOx catalyst increases as the NOx cleaning rate increases. Therefore, it is preferable to control the ammonia adsorption amount to be high in order to achieve the high cleaning rate of the NOx catalyst in a low temperature region. There is a limit in the ammonia adsorption amount capable of being adsorbed by the NOx catalyst and an adsorption limit value shown by a bold line (n) in FIG. 9 is dependent on the catalyst temperature.
When the NOx cleaning apparatus having the NOx catalyst of such a property is mounted to a vehicle, an operating state of the vehicle is continuously changed and a flow rate of exhaust gas exhausted from the internal combustion engine, an amount of NOx in exhaust gas and the catalyst temperature are significantly changed. Therefore, it is necessary to supply ammonia to the NOx catalyst in correspondence with the operating state of the vehicle. When supply of ammonia to the NOx catalyst cannot correspond to the operating state, for example, when the amount of supply of ammonia is deficient, the ammonia adsorption amount to the NOx catalyst becomes deficient and NOx cannot be reduced sufficiently. When the amount of supply of ammonia is excessively large and exceeds the adsorption limit value, a phenomenon referred to as ammonia slip, in which ammonia that has not been adsorbed to the NOx catalyst is exhausted to the atmosphere, is liable to be brought about.
The present invention resolves such a problem and it is an object thereof to provide a NOx cleaning apparatus and a NOx cleaning method for an internal combustion engine capable of effectively restraining ammonia slip from being brought about while maintaining a high NOx cleaning rate.
A NOx cleaning apparatus for an internal combustion engine, according to the invention, includes a NOx catalyst provided at an exhaust system of the internal combustion engine for selectively reducing NOx in an exhaust gas by adsorbing a reducing agent, a reducing agent supplying unit that supplies the reducing agent to the NOx catalyst, a NOx exhaust amount deriving unit that detects or estimates an exhaust amount of NOx exhausted from the internal combustion engine, an actual NOx cleaning rate deriving unit that derives an actual NOx cleaning rate by the NOx catalyst, a consumption amount deriving unit for deriving a consumption amount of the reducing agent absorbed to the NOx catalyst based on the NOx exhaust amount detected or estimated by the NOx exhaust amount deriving unit and the actual cleaning rate derived by the actual NOx cleaning rate deriving unit, an adsorption amount deriving unit that derives an actual adsorption amount of the reducing agent adsorbed to the NOx catalyst in accordance with the consumption amount of the reducing agent derived by the consumption amount deriving unit and an addition amount of the reducing agent by the reducing agent supplying unit, and a controlling unit that controls the reducing agent supplying unit in accordance with the actual adsorption amount derived by the adsorption amount deriving unit.
Therefore, according to the present invention, by detecting the actual adsorption amount of the reducing agent adsorbed to the NOx catalyst, a pertinent amount of the reducing agent can be supplied to constitute the adsorption amount of the reducing agent necessary for reducing the NOx exhaust amount and a high NOx cleaning rate and a reduction in the reducing agent, for example, ammonia slip can be made compatible with each other.